Conventionally, boosters of this type comprise a piston which consists of a rear tubular part and a skirt and which with the aid of a rolling diaphragm defines a front chamber permanently connected to a vacuum source and a rear chamber selectively connected to the front chamber or to the atmosphere by a valve means actuated by a control rod adapted to bear, by mean of a plunger, on one face of a reaction disk fastened to a push rod, the valve means comprising a valve closure member cooperating by an active part with a first valve seat formed on the plunger and with a second valve seat formed on the piston, the rear chamber being connected to the atmosphere via an annular space defined between the first and second valve seats when the plunger is driven into the piston.
Although their operation is satisfactory, boosters of this kind nevertheless have some disadvantages. Thus, during braking, the passage offered to atmospheric air towards the rear chamber is reduced, which slows down the movement of the air in the direction of the rear chamber and increases the response time of the booster. Various compromises have been proposed to make this response time acceptable for so-called "normal" braking when only the slowing down of the vehicle is desired. However, this response time is always much too long in the event of heavy braking for which rapid intervention of the desired.